The tendency in recent years towards higher mounting densities and larger capacities of large-scale integrated (LSI: Large Scale Integration) circuits are further reducing the circuit line widths needed for semiconductor devices.
Fabrication of semiconductor devices involves the use of photomasks or reticles (hereinafter, referred to collectively as masks) each having circuit patterns formed thereon. The circuit patterns on a mask are photolithographically transferred on to a wafer using a reduction projection exposure apparatus, often called a stepper, whereby the circuit patterns are formed on the wafer. An electron beam writing apparatus capable of writing fine patterns is used to manufacture the masks used to transfer the fine circuit patterns onto the wafer. This electron beam writing apparatus has inherently superior resolution, and can ensure greater depth of focus, thus allowing control over size fluctuations even with a difference in level.
Japanese Laid-Open Patent Publication No. Hei 9-293670 (1997) discloses a variable shape electron beam writing apparatus used for electron beam lithography. The pattern writing data for such apparatus is prepared by using design data (CAD data) of a semiconductor integrated circuit, for example, CAD data processed by a CAD system, wherein the CAD system divides the pattern.
For example, the pattern is divided into segments each the size of the maximum shot size, which is defined by the size of the electron beam. After this division of the pattern, the apparatus sets the coordinate positions and size of each shot and the radiation time. Pattern writing data is then produced which is used to shape the shot in accordance with the shape and size of the pattern or pattern segment to be written. The pattern writing data is divided corresponding to a strip-shaped frame (or main deflection region), and each frame is divided into sub-deflection regions. That is, the pattern writing data for the entire wafer has a hierarchical data structure in which data of each of a plurality of strip-shaped frames, which correspond to the main reflection regions, is divided into a plurality of data each representing one of the plurality of sub-reflection regions (smaller in size than the main deflection regions) in the frame.
The electron beam is scanned over each sub-deflection region by the sub-deflector at higher speed than it is scanned over each main deflection region; the sub-deflection regions are generally the smallest writing fields. When writing on each sub-deflection region, the shaping deflector forms a shot of a size and shape corresponding to the pattern or pattern segment to be written. Specifically, the electron beam emitted from the electron gun is shaped into a rectangular shape by a first aperture and then projected to a second aperture by the shaping deflector, resulting in a change in the shape and size of the beam. The electron beam is focused by an objective lens, then deflected by the sub-deflector and the main deflector and irradiated onto the mask mounted on the stage.
Furthermore, when a mask is irradiated with an electron beam, electrons reflected on the mask (reflected electrons) or electrons generated after entering the mask (secondary electron) proceed upward within an electron optical column.
FIG. 3 is a simulation of a path of reflected electrons having an energy value of 50 keV. Here, the simulation is performed only in a single direction in relation to each of exit angles of 10°, 30°, 50°, 70°, and 90°.
FIG. 4 is a simulation of a path of secondary electrons having an energy value of 100 eV. The simulation is performed only in a single direction in relation to each of exit angles of 10°, 30°, 50°, 70°, and 90°.
It should be noted that in FIGS. 3 and 4, the horizontal axis represents an x-axis, namely, a direction perpendicular to an electron beam axis. In addition, the vertical axis represents a Z-axis, namely, a direction parallel to the electron beam axis. Furthermore, an electromagnetic type lens, namely, a lens that generates a magnetic field by causing electric current to flow in a coil is used as an objective lens.
As can be seen from the simulation results shown in FIGS. 3 and 4, the reflected electrons or the secondary electrons perform a helical motion, coiling around the electron beam axis. Accordingly, the electron beam drifts under the influence of the reflected electrons or the secondary electrons, which results in irradiation to a position deviated from a target position.
The present invention has been made in view of the above problems. It is, therefore, an object of the present invention to provide an electron beam writing apparatus and an electron beam writing method capable of reducing the electron beam drift due to the reflected electrons or the secondary electrons.
Other challenges and advantages of the present invention are apparent from the following description.